Spray Method For Forming Shells For Prostheses

ABSTRACT

Shells for mammary prostheses and other devices are created by spraying a silicone dispersion onto a mandrel. Several coats of dispersion are applied with an interval for evaporation of solvent from the dispersion between application of coats. The shells created are uniform in thickness and have a desirably defect-free surface.

TECHNICAL FIELD

This invention relates to devices that are implantable in the human bodysuch as prostheses and tissue expanders.

BACKGROUND

Implantable prostheses are commonly used to replace or augment bodytissue. Such prostheses include a shell formed of an elastomericmaterial, e.g., silicone. The shell is filled with filling material suchas saline or some other liquid or a gel. The filling of the shellcommonly takes place after the shell is inserted through an incision.The shell includes a valve that can accept a filling tube that passesthrough the incision and is used to fill the shell with a suitableliquid or gel. Once the shell is filled to the desired degree, thefilling tube is removed and the incision is closed.

In the case of the female breast, it sometimes necessary to remove someor all of the mammary gland and surrounding tissue in order to treatbreast cancer. This surgery leaves a void that can be filled with animplantable prosthesis. The implant serves to support surrounding tissueand to maintain the appearance of the body. The restoration of thenormal appearance of the body has an extremely beneficial psychologicaleffect on post-operative patients, eliminating much of the shock anddepression that often follows extensive surgical procedures.

Implantable prostheses are also used more generally for restoring thenormal appearance of soft tissue in various areas of the body.

Tissue expanders generally resemble implantable prostheses except thatthey include a means for adding additional liquid or gel after thedevice had been inserted under the skin and the incision has beenclosed. After implantation the shell is gradually inflated using aliquid or gel, usually over a period of weeks, in order to expand theoverlying skin either so that a prosthesis can later be inserted or sothat skin can be generated for grafting. The liquid or gel is usuallyintroduced by means of a needle that pierces the skin and a self-sealvalve that is integral to the shell or that is remote from the shell andconnected to the shell by tubing.

Silicone shells for implantable prosthesis and tissue expanders aregenerally formed by dipping a suitably shaped mandrel into a siliconedispersion. The mandrel is withdrawn from the dispersion and the excesssilicone dispersion is allowed to drain from the mandrel. After theexcess dispersion has drained from the mandrel at least a portion of thesolvent is allowed to evaporate to stabilize the silicone coating. Theprocess is then repeated one or more times until a shell of the desiredthickness is formed. Because the flow of the silicone dispersion as itdrains from the mandrel depends on the shape and orientation of themandrel, the resulting shell can vary substantially in thickness. Inaddition, because dip casting requires a relatively large vat ofsilicone dispersion and because solvent evaporates from the siliconedispersion in the vat during the casting process, considerable siliconedispersion waste is created during dip casting.

SUMMARY

Methods for forming the shell of a body implant (e.g., a prosthesis ortissue expander) are described. The methods can also be used to makeother articles formed from an elastomeric material (e.g., a patch for animplant) and for applying a coating of an elastomeric material (e.g.,silicone) to a device to be implanted into the body (e.g., a pacemakeror implantable pump). The methods entail spraying a silicone dispersiononto a mandrel or other object. The silicone dispersion is sprayedusing, for example, a high volume, low pressure (HVLP) spray device or arotary atomizer or some other device that sprays the silicone dispersionat a low pressure. The methods described herein can be used, forexample, to create an implant shell that is both thinner and moreuniform in thickness than that which can be formed using traditional dipcasting techniques for forming the shell of body implant. In addition,the methods can be used to create a shell that varies in thickness in acontrolled manner. Further, allow the production of shells havingcomplex shapes that cannot be efficiently formed using traditional dipcasting methods. Thus, the methods can be used to make generallyspherical, cylindrical, crescent, half moon, rectangular solids, andcubic shell as well as other shapes, including shapes having edges,corners, recessed regions and other complex geometries. Shells createdby the methods of the invention have a generally smoother surface thanshells created by traditional dip casting methods.

In one aspect the method comprises: a) providing a mandrel, e.g., amandrel that is appropriately sized and shaped for form a desired shell,such as the shell of a body implant; b) spraying a silicone dispersiononto the mandrel until a desired thickness of silicone dispersion isformed on the mandrel; c) curing the silicone dispersion to form asilicone device; and d) removing the silicone device from the mandrel.In various embodiments: the silicone dispersion comprises hightemperature vulcanization (HTV) silicone, the silicone dispersioncomprises room temperature vulcanization (RTV) silicone.

In one aspect the method for creating a silicone shell comprises: a)providing a mandrel; b) applying a coating silicone dispersion to themandrel by spraying the silicon dispersion at low pressure; c) allowingevaporation of at least a portion of the solvent in the coating siliconedispersion; d) repeating steps b) and c) until a silicone shell having adesired thickness is formed; e) at least partially curing the siliconeshell; and f) removing the silicone shell from the mandrel.

In various embodiments: the silicone dispersion is sprayed onto themandrel using a high volume low pressure spray device; the siliconedispersion is sprayed onto the mandrel using a rotary atomizer; thesilicone dispersion is an HTV silicone dispersion; the siliconedispersion is an RTV silicone dispersion; at least two coats ofdispersion are applied to the mandrel; at least three coats ofdispersion are applied to the mandrel; at least four coats of dispersionare applied to the mandrel; at least five coats of dispersion areapplied to the mandrel; at least six coats of dispersion are applied tothe mandrel; the dispersion is sprayed by atomizing the dispersion usinga flow of air below 20 psi; the dispersion is sprayed by atomizing thedispersion using a flow of air below 10 psi; the shell is 0.00±0.004″thick; the shell is 0.012″±0.004″ thick; the shell is 0.014±0.004″thick; and the shell is 0.013±0.004″ thick.

In other embodiments, the method further comprising applying a coat ofsilicone dispersion to a portion of the mandrel to create a partial coatof dispersion; the mandrel includes an anterior surface and a posteriorsurface that meet at a perimeter region; the partial coat of dispersionis applied to the perimeter region of the mandrel; the mandrel has atleast one relatively planar region and at least on region that iscurved; the partial coat of dispersion is applied to the at least oneregion that is curved; the mandrel has a first region having a firstradius of curvature and a second region having a second smaller radiusof curvature; the region having the partial coat of dispersion isapplied to the region having a second, smaller radius of curvature; andat least one partial coat of dispersion is applied to at least a firstportion of the mandrel.

The invention also features a shell for a tissue expander formed byabove-described method and a shell for an implantable prosthesis made bythe above-described method.

The invention also features a prosthesis formed by a method comprisingproviding a shell formed by the above-described method and sealing theshell or sealing the shell while providing the shell with a fillingport.

For RTV silicone the silicone dispersion comprises 20-70% siliconesolids, 20-60% silicone solids, 20-50% silicone solids, 25-45% siliconesolids, 28-40% silicone solids, 28-39% silicone solids, 28-38% siliconesolids, 28-36% silicone solids, or 28-34% silicone solids. In certainembodiments the RTV dispersion contains 308-35% solids, preferably31%+/−3% silicone solids, 31%+/−2% silicone solids, or 31%+/−1% siliconesolids. In some embodiments the dispersion contains xylene or anothersuitable solvent.

For HTV silicone the silicone dispersion comprises 20-70% siliconesolids, 20-60% silicone solids, 20-50% silicone solids, 25-45% siliconesolids, 30-38% silicone solids, or 32-36% silicone solids. In certainembodiments the HTV dispersion contains 34% +/−3% silicone solids,34%+/−2% silicone solids, or 34%+/−1% silicone solids. In someembodiments the dispersion contains xylene or another suitable solvent.

For products filled with saline, the shell is formed of RTV silicone insome embodiments and is formed using a dispersion having a viscosity of700 to 820 centipoise. For products filled with silicone gel, the shellis formed of HTV silicone in some embodiments and is formed using adispersion having a viscosity of 500 to 600 centipoise.

The invention also features a silicone device formed by: a) providing amandrel; b) spraying a silicone dispersion onto the mandrel until acoating of the desired thickness is formed on the mandrel; c) curing thecoating of silicone dispersion on; and d) removing the silicone devicefrom the mandrel.

The method can be used to form an HTV silicone shell with a thicknessof: 0.013″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″, ±0.0025″ or±0.002″; 0.012″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″, ±0.0025″ or±0.002″; 0.011″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″, ±0.0025″ or±0.002″; 0.010″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″, ±0.0025″ or±0.002″; or 0.009″±0.007″, 0.006″, ±0.005″, ±0.004″, ±0.003″, ±0.0025″or ±0.002″. In certain embodiments the method is used to create asilicone shell that varies in thickness by no more than 0.006″, 0.005″,0.004″, 0.003″, 0.0025″ or 0.002″.

The method of the invention is used to form an RTV shell with athickness of: 0.015″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″,±0.0025″ or ±0.002″; 0.014″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″,±0.0025″ or ±0.002″; 0.013″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″,±0.0025″ or ±0.002″; 0.012″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″,±0.0025″ or ±0.002″; 0.011″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″,±0.0025″ or ±0.002″; 0.010″±0.007″, ±0.006″, ±0.005″, ±0.004″, ±0.003″,±0.0025″ or ±0.002″; or 0.009″±0.007″, ±0.006″, ±0.005″, ±0.004″,±0.003″, ±0.0025″ or ±0.002″. In certain embodiments the method is usedto create a silicone shell that varies in thickness by no more than0.006″, 0.005″, 0.004″, 0.003″, 0.0025″ or 0.002″.

In some embodiments the mandrel is sprayed with 3, 4, 5, 6, 7 or morecoats of silicone dispersion. In some embodiments solvent is allowed toevaporate from the dispersion for 1, 2, 3, 4, 5, 10, 15, 20 or moreminutes between coats. In some embodiments the mandrel is rotated duringapplication of the one or more coats of silicone dispersion.

In certain embodiments the first coat of dispersion on the mandrelcontains less dispersion than subsequent coats. For example, the firstcoat is formed by two passes of the spray head over each portion of themandrel and subsequent coats are formed by three passes of the sprayhead over each portion of the mandrel. Thus, the first coat containsless material that subsequent coats and is generally thinner thansubsequent coats.

In various embodiments: the mandrel is appropriately sized and shapedfor forming an implantable prosthesis; the mandrel is appropriatelysized and shaped for forming an implantable mammary prosthesis; themandrel is appropriately sized and shaped for forming a tissue expander;the mandrel is and shaped for forming a round body implant; the mandrelis shaped for forming a crescent-shaped body implant; the mandrel isappropriately sized and shaped for forming an oval body implant.

Also described is a prosthesis formed by a method comprising: a)providing a mandrel that is appropriately sized and shaped for forming abody implant; b) applying a coating silicone dispersion to the mandrelby spraying the silicone dispersion at low pressure; c) allowingevaporation of at least a portion of the solvent in the coating siliconedispersion; d) repeating steps b) and c) until a silicone shell having adesired thickness is formed; e) at least partially curing the siliconeshell; f) removing the silicone shell from the mandrel; and g) applyinga seal to the shell. In some embodiments, the shell is shaped and sizedfor use as a mammary prosthesis.

Also featured is a tissue expander formed by a method comprising: a)providing a mandrel that is appropriately sized and shaped for forming abody implant; b) applying a coating silicone dispersion to the mandrelby spraying the silicon dispersion at low pressure; c) allowingevaporation of at least a portion of the solvent in the coating siliconedispersion; d) repeating steps b) and c) until a silicone shell having adesired thickness is formed; e) at least partially curing the siliconeshell; f) removing the silicone shell from the mandrel; g) providing theshell with a filling port; and h) sealing the shell. In variousembodiments: the mandrel is appropriately sized and shaped for formingan implantable prosthesis; the mandrel is appropriately sized and shapedfor forming an implantable mammary prosthesis; the mandrel isappropriately sized and shaped for forming a tissue expander; isappropriately sized and shaped for forming a round body implant; themandrel is appropriately sized and shaped for forming a crescent-shapedbody implant; the mandrel is appropriately sized and shaped for formingan oval body implant; the mandrel is appropriately sized and shaped forforming a testicular implant; the shell is shaped and sized for use as amammary tissue expander.

The method for forming a prosthesis can further include: filling theprosthesis with a liquid; filling the prosthesis with a gel; filling theprosthesis with a saline solution and filling the prosthesis withsilicone filler.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view of the anterior face of the shell of a mammaryprosthesis.

FIG. 2 is schematic drawing of a cross-section of the shell of a mammaryprosthesis taken along A-A′ in FIG. 1.

DETAILED DESCRIPTION

Described below are methods for forming a silicone articles, e.g., ashell for soft tissue prostheses. The methods entail spraying a siliconedispersion onto a suitably shaped mandrel or an object to be coatedusing, for example, a high volume, low pressure (HVLP) spray device or arotary atomizer. The methods of the invention allow one to control thethickness of shell, permitting the creation of shells that are of auniform thickness as well as shells that vary in thickness in acontrolled manner.

The spraying of the silicone dispersion can be accomplished using HVLPsystems. The spraying can be accomplished by atomizing the dispersionusing relatively low pressure air. The spraying can also be accomplishedby rotary atomization. In rotary atomization the dispersion is feed to acone or bell spinning at, e.g., 10,000-40,000 RPM. The dispersion isfeed to the cone or bell at low pressure (e.g., the dispersion pressureis at or below 30 psi, below 20 psi, 10±4 psi or between 0.1 and 10psi). Rotary atomization forms a very smooth, uniform layer of siliconethat has very few pits or other imperfections. Rotary atomizers aredescribed well known in the art (see, e.g., U.S. Pat. Nos. 5,633,306 and4,887,770). Rotary atomizers are available from Ransburg Corporation(Toledo, Ohio) and other suppliers. The spray head (cone or bell) of arotary atomizer can be controlled by a programmable computer so that thespray head makes the same number of passes over all portions of themandrel or so that the spray head makes more passes over some portionsof the mandrel than other portions.

The methods of the invention have numerous advantages over thetraditional dip casting technique used to create shells for soft tissueimplants. In dip casting, a shell is formed by first dipping a mandrelinto a silicone dispersion. Once the mandrel is removed from thedispersion, excess dispersion is allowed to drain from the mandrel.Solvent is allowed to evaporate from the coating of silicone dispersionremaining on the mandrel until the coating is sufficiently stabilized toallow the mandrel to be dipped into the dispersion again. This processis repeated until a shell of the desired thickness is created. Becausethe flow of the silicone dispersion as it drains from the mandreldepends on the shape and orientation of the mandrel, the resulting shellcan vary substantially in thickness. For example, the thickness of theshell of a mammary prosthesis formed by dip casting might vary from0.009 to 0.024″ from one region to another, a variation of more than100%. Importantly, the thickness of the various regions of the shell isdictated for the most part by the shape of the mandrel and theorientation of the mandrel during the period that the siliconedispersion is draining from the mandrel.

FIG. 1 is a plan view of a mammary prosthesis. The anterior face 100 isthe outward face when the prosthesis is inserted under the skin of apatient's chest. The prosthesis includes an upper pole region 120 (i.e.,the upper half of the shell when the prosthesis recipient is standing)and a lower pole region 130 (i.e., the lower half of the shell when theprosthesis recipient is standing). The region 135 where the anteriorface 100 meets the posterior face (not shown) is sometime referred to asthe radius, perimeter or edge FIG. 2 is a cross-sectional view of amammary prosthesis taken along A-A′ in FIG. 1. This type of shell has ashape that resembles that of a human breast in that it is fuller in thelower portion than in the upper portion. The shell has an anterior faceregion 100, a posterior face region 110 (i.e., the face placed againstthe patient's chest wall when the prosthesis is implanted), an upperpole region 120 and a lower pole region. The region where the posteriorand anterior face or surface meet 135, sometimes referred to as theradius, perimeter or edge, is relatively curved compared to theposterior face and to some extent even the anterior face. In the shelldepicted here, the radius of curvature of the perimeter in the upperpole region is relatively small. The radius of curvature of theperimeter in the lower pole region is larger. As noted above the shelldepicted in FIG. 2 has a shape that more closely resembles the humanbreast than other types of shells, e.g., shells that are symmetricallydome shaped.

In dip casting the mandrel is held such that the portion correspondingto the anterior face of the shell faces downward. The bracket or rodused to hold the mandrel as it is dipped into the dispersion extendsoutward from that portion of the mandrel corresponding to the posteriorface of the shell. As dispersion drains from the mandrel after it iswithdrawn from the dispersion it can be appreciated that it drainsrelatively rapidly from the perimeter region. As a result, the shell inthe perimeter region tends to be thinner than the shell in the anteriorface region. For this reason, additional dips are required to create anadequate shell thickness in the perimeter region. This can result inshell that is thicker than desired in the anterior face region. Inaddition, because the dispersion flows over the mandrel and onto theportion corresponding to the posterior face of the shell, the posteriorface region is generally thicker than the shell anterior face region andcan be thicker than desired.

As can be seen, the radius of curvature in upper pole transition region180 between the anterior face region 100 and posterior face region 110is quite small. It can be desirable for the shell in this region to bethicker than, for example, the anterior face region 100, which should berelatively thin to preserve a natural appearance when implanted in apatient. The method of the invention allows for creating a thicker shellin upper pole perimeter or radius region while maintaining a thinnershell in other regions, e.g., the anterior face region 100 by simplycontrolling the spray head to apply additional coats of siliconedispersion in the upper pole perimeter or radius region.

In contrast, the methods of the invention can be used to create a shellfor a mammary implant or other that varies in thickness from, e.g.,0.009″±0.003″ on the faces (e.g., the anterior and posterior faces) to0.024″±0.003 on the edges or comers where two or more faces meet. Themethod can also be used to create shells with greater variation inthickness. Thus, as described in greater detail below, the methods ofthe invention can also be used to create shells that vary in diameter ina controlled manner, i.e., a manner that is not dictated by the shape ofthe mandrel.

In one embodiment of the invention, a silicone dispersion is applied toa mandrel using a robotically controlled rotary atomizer spray head. Therotary atomizer can operate electrostatically, wherein there is adifference in electrical potential between the silicone and the mandrel.For example, a charge can be applied to the dispersion and the mandrelcan be grounded, such that the atomized silicone dispersion is attractedthrough electrical forces to the mandrel. The spray head makes one ormore (e.g., 1, 2, 3, 4, 5, or more) passes over an area of the mandrelto apply one coat of silicone to the entire mandrel or desired portionthereof. The spray head or the mandrel or both can be moved during theapplication of a coat of silicone dispersion. A number of coats areapplied depending on various factors such as the type of silicone used(HTV or RTV), the percent of silicone solids in the dispersion, and thedesired thickness of the shell. Between the application of each coat ofsilicone dispersion solvent is allowed to evaporate so that the coat ofsilicone dispersion is at least somewhat stabilized prior to theapplication of the next coat of silicone dispersion. Thus, there is aninterval of several minutes (e.g., 2, 3, 4, 5, 6, 10, 12, 15, 20. 25minutes or more) between the application of one coat of dispersion(which may require 1, 2, 3, 4 or more passes of the spray head over allor a portion of the mandrel) and the next coat of dispersion. Once thedesired number of coats of silicone dispersion have been applied, theshell is allowed to fully cure and it is then removed from the mandrel.Because there is no need for excess dispersion to drain from themandrel, each coat applied is quite uniform in thickness, i.e., it isnot significantly thicker in the posterior face region than it is in theanterior face region.

In applying the dispersion, some portions of the mandrel can receivemore coats of dispersion than other portions of the mandrel. Forexample, in the case of a mammary implant the perimeter region canreceive more coats of dispersion than the anterior face. In general itcan be desirable to apply more coats of material (resulting in a thickershell) in those regions that correspond to an edge or corner or thatotherwise have a smaller radius of curvature than in those regions thatare relatively planar.

To provide shells or other devices with one or more regions that arethicker than one or more other regions, it can be desirable to applymore coats of silicone dispersion to some portions of the mandrel thanother portions. Thus, one can apply a partial coat of dispersion, i.e. acoat that does not cover the entire mandrel, but instead covers only aportion of the mandrel. Additional partial coats can be applied to themandrel so that one or more portions of the mandrel have 1, 2, 3, 4 ormore additional coats of dispersion compared to other portions of themandrel. Moreover, additional partial coats can be applied to two ormore different regions of the mandrel. Thus, the entire mandrel canreceive a total 4 coats of dispersion, one portion can receive a totalof 5 coats of dispersion and yet another portion can receive a total of6 coats of dispersion.

It is desirable to use a silicone dispersion having a defined percentsolids so that the thickness of the layer of silicone dispersion appliedin each coat is relatively predictable. This is in contrast totraditional dip casting methods in which it is desirable to use asilicone dispersion having a defined viscosity. A silicone dispersionhaving a defined viscosity is desirable in dip casting because theformation of the shell is highly dependent on the bulk flowcharacteristics of the silicone dispersion. In many instances therelationship between viscosity and percent solids is not predictable.Thus, for shells produced by spray techniques it is desirable to use adispersion with a defined percent solids rather than a definedviscosity.

EXAMPLE 1 RTV Silicone Shell

An appropriately shaped mandrel for forming the shell of a mammaryprosthesis is arranged approximately 3″ to 6″ inches from a roboticallycontrolled spray head of rotary atomizer spray device (e.g., theAerobell RMA-101; ITW Ransburg, Inc.; Toledo Ohio). The mandrel isarranged so that the portion of the mandrel corresponding to theposterior face of the shell is facing downward. The mandrel is held on arod that extends from the downward facing portion of the mandrel andthis rod is arranged so as to allow the mandrel to be rotated on theaxis of the rod. The mandrel and the spray head are contained in acabinet where the air temperature is held at 90° F.±10° F. and about35-45% relative humidity. A RTV silicone dispersion having 31% (±2%)solids in a xylene dispersion is applied to the mandrel using a sprayhead that travels in an arc from above the mandrel (90° above thehorizontal) to below the mandrel (90° below horizontal) completing onepass from above the mandrel (the portion of the mandrel corresponding tothe anterior face of the shell) to below the mandrel (the portion of themandrel corresponding to the posterior face of the shell) in about 5seconds as the mandrel is rotated at about 20 rpm. The spray head issupplied by a ⅜″ diameter supply line and the dispersion is pumpedthrough the line at 8-20 p.s.i. The dispersion can be electricallycharged and the mandrel grounded in order to electrostatically attractthe silicone dispersion to the mandrel. However, since RTV siliconedispersion is not particularly conductive, similar results can beachieved with and without charging the dispersion. To apply one coat,the spray head travels from above the mandrel to below the mandrel andreturns to above the mandrel during which time the mandrel rotates about5 full revolutions. The application of a single coat of dispersion takesabout 15 seconds. Solvent is allowed to evaporate from the siliconedispersion coated on the mandrel for 10 to 20 minutes in adevolatilization step. This process is repeated so that 4 to 5additional coats of dispersion are applied in the same manner with adevolatilization step occurring between each coat. The final thicknessof the shell is 0.014″±0.002″. The process can be automated by mountinga number of mandrels on a track that passes the mandrels by the sprayhead. The spray head can move with the line or the line can pause with amandrel positioned near the spray head. After the final coat has beenapplied, the shell is cured by placing the coated mandrel in an oven setto 150° F. for at least 20 to 30 minutes. The shell is stripped from themandrel and can be used in a tissue expander or prosthesis.

EXAMPLE 2 HTV Silicone Shell

An appropriately shaped mandrel for forming the shell of a mammaryprosthesis is arranged approximately 3″ to 6″ inches from a roboticallycontrolled spray head of rotary atomizer spray device (e.g., theAerobell RMA-101; ITW Ransburg, Inc.; Toledo Ohio). The mandrel isarranged so that the portion of the mandrel corresponding to theposterior face of the shell is facing downward. The mandrel is held on arod that extends from the downward facing portion of the mandrel andthis rod is arranged so as to allow the mandrel to be rotated on theaxis of the rod. The mandrel is heated to about 145° F. by an infraredheating device that allows the surrounding air temperature to besignificantly lower. The mandrel and the spray head are contained in acabinet where the air temperature is held at 115° F.±10° F. and about35-45% relative humidity. A HTV silicone dispersion having 31% (±2%)solids in a xylene dispersion is applied to the mandrel using a sprayhead that travels in an arc from above the mandrel (90° above thehorizontal) to below the mandrel (90° below horizontal) completing onepass from above the mandrel (the portion of the mandrel corresponding tothe anterior face of the shell) to below the mandrel (the portion of themandrel corresponding to the posterior face of the shell) in about 5seconds as the mandrel is rotated at about 20 rpm. The spray head issupplied by a ⅜″ diameter supply line and the dispersion is pumpedthrough the line at 8-20 psi. Shaping air can be employed to shape thestream of silicone dispersion. The dispersion can be electricallycharged (e.g., by applying 80,00 to 100,000 volts at low amperage) andthe mandrel grounded in order to electrostatically attract the siliconedispersion to the mandrel. Because HTV silicone dispersion isconductive, superior results can be achieved by charging the dispersion.To apply one coat, the spray head travels from above the mandrel tobelow the mandrel and returns to above the mandrel during which time themandrel rotates about 5 full revolutions. The application of a singlecoat of dispersion takes about 15 seconds. Solvent is allowed toevaporate from the silicone dispersion coated on the mandrel for 10 to20 minutes in a devolatilization step. This process is repeated so that3 to 4 additional coats of dispersion are applied in the same mannerwith a devolatilization step occurring between each coat. The finalthickness of the shell is 0.012″±0.002″. The process can be automated bymounting a number of mandrels on a track that passes the mandrels by thespray head. The spray head can move with the line or the line can pausewith a mandrel positioned near the spray head. After the final coat hasbeen applied, the shell is cured by placing the coated mandrel in anoven set to 325° F. for at least 55 to 65 minutes. The shell is strippedfrom the mandrel and can be used in a tissue expander or prosthesis.

EXAMPLE 3 Textured Shell

A textured shell can be created by applying solid particles of siliconeto a shell between coats of silicone dispersion. A shell of the desiredthickness is created as described in Example 1 or Example 2. The shellis partially or fully cured by heating. Alternatively, the shell is notcured at all. The tackiness of an uncured or partially cured siliconeshell fosters adhesion of the solid particles. Particles of fully curedground silicone gum rubber having an average diameter of 50 to 1500microns are applied to the surface of the shell, e.g., by dipping themandrel into a bed of particles or by blowing particles onto the shell.Because the surface of the shell is tacky and because the siliconeparticles have a static charge, the silicone particles adhere readily.The silicone particles can be applied at a density that allows almostcomplete coverage of the shell or partial coverage. Thus, the particlescan be applied relatively densely so that there is little exposed shellor they can be applied relatively sparsely so that there is considerableexposed shell. In addition the particles can be applied to only aportion of the shell, e.g., the particles can be applied only to theposterior face of the shell. The shell is heated (at 250-325° F. for 30to 60 minutes in the case of HTV silicone) to partially cure or gel thesilicone layer to which the particles are adhered. Particles that do notadhere to the shell are removed by gently blowing air over the surfaceof the shell. The shell is then sprayed again with a silicone dispersion(e.g., HTV silicone diluted to, e.g., 10 to 13% solids with xylene,toluene, tetrahydeofuran or some other suitable solvent) to apply aparticle coating layer that envelops the applied particles. The particlecoating layer is partially cured or gelled by, for example, heating theshell to 250-325° F. for 30 to 60 minutes in the case of particlescoated with an HTV silicone dispersion. The application of particles,curing, coating with silicone dispersion and curing is repeated two moretimes for a total of three application of particles and threeapplications of silicone dispersion. Finally, the shell is fully cured.

Silicone particles can be created, for example, from fully curedsilicone that is cryogenically ground to yield particles ranging in sizefrom 50-600 microns in diameter, e.g., 50-100, 100-200, 200-300,300-400, 400-500, or 500-600 microns in diameter. For example, ¼ to ½inch thick silicone sheets can be cryogenically ground to yield suitableparticles. In some cases the particles are subsequently size selected,e.g., by sieving, such that the particles have a selected average size,e.g., 200-300 microns. In some cases the size selection is limited toremoving very small particles and very large particles.

As noted above, the silicone particles can be applied several times. Insome cases that average size of the silicone particles applied willvary. Thus, the particles applied in the first application of particlescan have a first average size and the particles applied in the secondapplication of particles can have a second average size. If there is athird application of particles, the particles in this third applicationcan have a third average particle size. Thus, the particles used in eachapplication can have a different average particle size. In some cases,several of the applications of particles can have the same or verysimilar average particle size. In some cases a given application ofparticles can include two different groups of particles, one having afirst average particle size and the other having a second, differentaverage particle size.

The surface created on the shell is microporous. There are cavities,overhangs, bridges and passageways. However, because each layer ofparticles is enveloped in silicone, the surface is relatively smoothlymodeled. Because the surface of the shell includes cavities, overhangsand passageways, upon implantation, tissue will grow into the cavities,beneath the overhangs and through the passageways. In this manner, thetissue is engaged with the implant. This engagement secures the implant.

EXAMPLE 4 Shell with Varying Thickness

Implantable mammary prostheses generally have a relatively flatposterior face that is placed against the patient's chest and a domedanterior face. It is often desirable for the region where the anteriorface meets the posterior face, the perimeter region, to have arelatively small radius of curvature, particularly at the upper pole ofthe prosthesis, i.e., the portion of the prosthesis that is uppermostwhen the patient is standing. A relatively small radius of curvature inthe transition between the anterior face and the posterior face in theupper pole of the prosthesis is desirable because it permits arelatively smooth transition between the mammary tissue and the implantwhen the prosthesis is implanted. However, a small radius of curvatureis sometimes associated with the appearance of creases that extendinward from the perimeter of the prosthesis in the region of relativelysmall radius. This is sometimes referred to as a scalloping effect.Scalloping tend to occur when the prosthesis is filled with fluid andthe patient is upright such that the weight of the filling material ispulling downward on the prosthesis. The creases often appear on theanterior face of the prosthesis. This is aesthetically undesirable asthe creases can sometimes be discerned through the overlying skin of thepatient.

The spray methods described herein prosthesis can be used to create ashell of non-uniform thickness, e.g., a shell in which the averagethickness of the shell in the region where the posterior and anteriorfaces meet is greater, e.g., at least 25%, 50%, 75%, 100% greater thanthe thickness of the shell in the region of the anterior face. Thisreinforcement can reduce or eliminate undesirable scalloping effects. Insome cases, the average thickness of the shell in the region ofposterior face is also greater, e.g., at least 25%, 550%, 75%, 100%greater than the thickness of the shell in the region of the anteriorface. Moreover, because the reinforcement can be confined to theperimeter (or radius) of the shell and, optionally, posterior face ofthe shell, it is possible for nearly the entire anterior face of theprosthesis to be formed of relatively thin material. This preserves anatural appearance of the prosthesis when it is implanted.

A shell having varying thickness can be created by spraying more coatsof dispersion on one or more some parts of the mandrel or by applyingthe dispersion more heavily to one or more parts of the mandrel, e.g.,by adjust spray volume or the speed at which the mandrel is rotated asthe dispersion is applied. A shell having variable thickness can havecreated such that: the average thickness of the shell in the perimeterregion is about 0.02 to 0.08 inches, the average thickness of the shellin the entire perimeter region is greater than the average thickness ofthe shell in the anterior face, the average thickness of the posteriorface of the shell is about 0.02 to 0.08 inches, the average thickness ofthe anterior face of the shell is less than about 0.04 inches, or theaverage thickness of the anterior face of the shell is less than about0.03 inches.

The invention also features a kit comprising a prosthesis of theinvention and a means for filling the prosthesis with a liquid or a gel.In a preferred embodiment, the filling means comprises a syringe and atube adapted to connect the syringe to the filling port of the shell ofthe prosthesis.

The invention further features a method for forming a prosthesis, themethod comprising: (a) providing a mandrel adapted for dip molding amammary prosthesis, the mandrel having an anterior face, a posteriorface and a perimeter region where the anterior face and the posteriorface meet; (b) masking both the anterior face and the posterior face ofthe mandrel while leaving at least a portion of the perimeter regionfree of masking; (c) dipping the masked mandrel into a siliconedispersion

OTHER EMBODIMENTS

A soft tissue prosthesis can have any desired shape, e.g., the shell ofthe prosthesis can be circular, oval, or crescent shaped. The prosthesiscan have a single lumen or multiple lumens. It can be formed of siliconerubber, a laminate of various forms of silicone, silicone copolymers,polyurethane, and various other elastomers in various combinations.Various materials are described in U.S. Pat. Nos. 4,592,755 and4,205,401.

To form a prosthesis from the shell, e.g., a shell formed of HTVsilicone, the opening in the posterior face of the shell is sealed usinga patch comprising a vulcanized layer of silicone sheeting and anunvulcanized layer of silicone. The patch is shaped and sized to besomewhat larger than the opening in the posterior face of the shell. Thepatch is positioned inside the shell such that the unvulcanized layer ofthe patch faces outward and the perimeter of the patch overlaps the edgeof the shell surrounding the opening. The assembly is compressed eitherbetween hot platens at, e.g., 325° F. and 60 p.s.i. or platens at roomtemperature and 60 p.s.i. for about two to three minutes. The patchedshell is then cured in an oven at 325° F. for about one half hour tocure fully.

The shell can be filled with a fluid or gel. In addition, an amount ofsolid material can be combined with the fluid or gel to adjust thedensity or compressibility of the filling.

Elastomers other than silicone may be used. Thus, the mandrel can besprayed with a dispersion of any elastomer.

The prosthesis of the invention can be provided as a kit with a shelland a means for filling the shell, e.g., a syringe. The kit can furtherinclude an adapter tube for connecting the syringe to the filling portof the shell.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.

1. A method for creating a silicone shell, the method comprising: a)providing a mandrel suitably sized and shaped for forming a desiredshell; b) applying a coating silicone dispersion to the mandrel byspraying the silicon dispersion at low pressure to create a shell havinga desired thickness; c) at least partially curing the silicone shell;and d) removing the silicone shell from the mandrel.
 2. A method forcreating a silicone shell, the method comprising: a) providing a mandrelsuitably sized and shaped for forming a desired shell; b) applying acoating silicone dispersion to the mandrel by spraying the silicondispersion at low pressure; c) allowing evaporation of at least aportion of the solvent in the coating silicone dispersion; d) repeatingsteps b) and c) until a silicone shell having a desired thickness isformed; c) at least partially curing the silicone shell; and d) removingthe silicone shell from the mandrel.
 3. The method of claim 1 whereinthe silicone dispersion is sprayed onto the mandrel using a high volumelow pressure spray device.
 4. The method of claim 1 wherein the siliconedispersion is sprayed onto the mandrel using a rotary atomizer.
 5. Themethod of claim 1 wherein the silicone dispersion is an HTV siliconedispersion.
 6. The method of claim 1 wherein the silicone dispersion isan RTV silicone dispersion.
 7. The method of claim 2 wherein at leasttwo coats of dispersion are applied to the mandrel.
 8. The method ofclaim 2 wherein at least three coats of dispersion are applied to themandrel.
 9. The method of claim 2 wherein at least four coats ofdispersion are applied to the mandrel.
 10. The method of claim 1 whereinat least five coats of dispersion are applied to the mandrel.
 11. Themethod of claim 1 wherein the dispersion is sprayed by atomizing thedispersion using a flow of air below 20 psi.
 12. The method of claim 1wherein the dispersion is sprayed by atomizing the dispersion using aflow of air below 10 psi.
 13. The method of claim 1 wherein the shell is0.00±0.004″ thick.
 14. The method of claim 1 wherein the shell is0.012″±0.004″ thick.
 15. The method of claim 1 wherein the shell is0.014±0.004″ thick.
 16. The method of claim 1 wherein the shell is0.013″±0.004″ thick.
 17. The method of claim 1 further comprisingapplying a coat of silicone dispersion to a portion of the mandrel tocreate a partial coat of dispersion.
 18. The method of claim 17 whereinthe mandrel includes an anterior surface and a posterior surface thatmeet at a perimeter region.
 19. The method of claim 18 wherein thepartial coat of dispersion is applied to the perimeter region of themandrel.
 20. The method of claim 17 wherein the mandrel has at least onerelatively planar region and at least on region that is curved.
 21. Themethod of claim 20 wherein the partial coat of dispersion is applied tothe at least one region that is curved.
 22. The method of claim 20wherein the mandrel has a first region having a first radius ofcurvature and a second region having a second smaller radius ofcurvature.
 23. The method of claim 22 wherein the region having thepartial coat of dispersion is applied to the region having a second,smaller radius of curvature.
 24. The method of claim 17 wherein at leastone partial coat of dispersion is applied to at least a first portion ofthe mandrel.
 25. The method of claim 1 wherein the mandrel isappropriately sized and shaped for forming an implantable prosthesis.26. The method of claim 25 wherein the mandrel is appropriately sizedand shaped for forming an implantable mammary prosthesis.
 27. The methodof claim 1 wherein the mandrel is appropriately sized and shaped forforming a tissue expander.
 28. The method of claim 1 wherein the mandrelis appropriately sized and shaped for forming a round body implant. 29.The method of claim 1 wherein the mandrel is appropriately sized andshaped for forming a crescent-shaped body implant.
 30. The method ofclaim 1 wherein the mandrel is appropriately sized and shaped forforming an oval body implant.
 31. A shell for a tissue expander formedby the method of claim
 1. 32. A shell for an implantable prosthesisformed by the method of claim
 1. 33. A prosthesis formed by a methodcomprising: a) providing a mandrel that is appropriately sized andshaped for forming a body implant; b) applying a coating siliconedispersion to the mandrel by spraying the silicon dispersion at lowpressure; c) allowing evaporation of at least a portion of the solventin the coating silicone dispersion; d) repeating steps b) and c) until asilicone shell having a desired thickness is formed; e) at leastpartially curing the silicone shell; f) removing the silicone shell fromthe mandrel; and g) applying a seal to the shell.
 34. The prosthesis ofclaim 33 wherein the shell is shaped and sized for use as a mammaryprosthesis.
 35. A tissue expander formed by a method comprising: a)providing a mandrel that is appropriately sized and shaped for forming abody implant; b) applying a coating silicone dispersion to the mandrelby spraying the silicon dispersion at low pressure; c) allowingevaporation of at least a portion of the solvent in the coating siliconedispersion; d) repeating steps b) and c) until a silicone shell having adesired thickness is formed; e) at least partially curing the siliconeshell; f) removing the silicone shell from the mandrel; g) providing theshell with a filling port; and h) sealing the shell.
 36. The prosthesisof claim 33 wherein the mandrel is appropriately sized and shaped forforming an implantable prosthesis.
 37. The prosthesis of claim 33wherein the mandrel is appropriately sized and shaped for forming animplantable mammary prosthesis.
 38. The prosthesis of claim 33 whereinthe mandrel is appropriately sized and shaped for forming a tissueexpander.
 39. The prosthesis of claim 33 wherein the mandrel isappropriately sized and shaped for forming a round body implant.
 40. Theprosthesis of claim 33 wherein the mandrel is appropriately sized andshaped for forming a crescent-shaped body implant.
 41. The prosthesis ofclaim 33 wherein the mandrel is appropriately sized and shaped forforming an oval body implant.
 42. The prosthesis of claim 33 wherein atleast a portion of the external surface of the shell is textured. 43.The tissue expander of claim 34 wherein the shell is shaped and sizedfor use as a mammary tissue expander.
 44. The tissue expander of claim34 wherein at least a portion of the external surface of the shell istextured.
 45. The method of claim 1 wherein the silicone dispersioncontains 10% to 50% silicone solids.
 46. The method of claim 1 whereinthe silicone dispersion contains 20% to 40% silicone solids.
 47. Themethod of claim 1 wherein the silicone dispersion contains 25% to 35%silicone solids.
 48. The method of claim 1 wherein the dispersioncontains 28% to 32% solids.
 49. The method of claim 33 furthercomprising filling the prosthesis with a liquid.
 50. The method of claim33 further comprising filling the prosthesis with a gel.
 51. The methodof claim 33 further comprising filling the prosthesis with a salinesolution.
 52. The method of claim 33 further comprising filling theprosthesis with silicone filler.